The present invention relates to the construction of electrolysis cells for the production of aluminum by the Hall-Heroult process. More particularly, the present invention relates to a shield which is intended to prevent seepage of the electrolyte constituents into the subcathodic region.
Cells for the production of aluminum by the Hall-Heroult process generally consist of a metallic housing, the base of which is coated with a refractory and insulating material, this housing supporting the carbonaceous blocks which form the cathode on which the liquid aluminum is deposited. Sealed contact between adjacent cathodic blocks and between the cathodic blocks and the walls of the housing is generally assured by a carbonaceous paste or coating which is based on pitch and coke or anthracite.
After the initial heating of the cell, fissures such as in the paste may be formed by the action of differential expansion, through which fissures the molten electrolyte, consisting for the most part of cryolite, begins to seep. This cryolite seepage tends to erode the insulating properties of the underlying refractory material. Liquid aluminum may also seep through the same channels and attack the insulating refractories which are located between the carbonaceous cathode and the metallic housing. These refractories are generally composed of silica or silicates which are reducible by the liquid aluminum.
Moreover, particularly during the initial months of operation of the electrolysis cells, there is a gradual impregnation of the carbonaceous coating of the cell by the constituent elements of the bath and in particular by sodium and fluorine. When these sodium-containing and fluorine-containing materials have penetrated the carbonaceous coating they may attack the underlying insulating coating.
As a result of this erosion of the thermal insulators by these materials, the thermal insulation of the cell is reduced and thermal losses are increased. This is not only detrimental to the energy consumption per ton of aluminum which is produced, but it also means that it is difficult to find a satisfactory thermal balance for a series of cells which are of different ages.
In order to limit the effect of seepage and impregnation, it was proposed in U.S. Pat. No. 4,175,022 corresponding to French Pat. No. A-2,388,901, to place a protective steel layer below the insulating material. For a shield of this type to be effective, however, the patentee stipulated that it would have to be relatively thick, more than 5 mm, and would have to be continuous. Moreover, the periphery thereof would have to be maintained at a sufficiently low temperature (from 500.degree.-600.degree. C.) to prevent sodium-containing and fluorine-containing seepages (cryolite) from deforming it.
Under these conditions, a monoblock thick screen suffers from two major disadvantages:
the difference in temperature between the center (about 900.degree. C.) and the periphery (about 500.degree. C.) of the shield gives rise to a considerable thermal flow towards the periphery of the vat, thereby unacceptably modifying its thermal operations and degrading energy consumption; and
this difference in temperature causes considerable differential thermal expansion between the center of the shield and its periphery and this gives rise to detrimental deformation of the coating and the cathode.